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Damage Control: A $360 Part That Can Save You Thousands

Thanks to its compacted graphite iron block, reverse-flow aluminum cylinder heads, variable-geometry turbocharger, and 30,000-psi common-rail fuel injection system, Ford's 6.7L Power Stroke is an impressive engine. It's been available with at least 400 hp and 800 lb-ft of torque right off the showroom floor since 2011, and current '19 models come with an incredible 450 hp and 935 lb-ft of torque. For 2020, Ford's next-generation V-8 diesel is rumored to pack 1,000 lb-ft or more from the factory. However, just as it has since its inception, the 6.7L Power Stroke harbors an Achilles' heel that can cost you as much as $10,000 in repairs.

The Bosch CP4.2 high-pressure fuel pump at the heart of the common-rail system has a design flaw—and it's a shortcoming that can cause the pump to self-destruct and take the fuel rails and injectors out with it. Aside from replacing the CP4.2 pump altogether, the folks at S&S Diesel Motorsport have come up with a CARB-compliant and 50-state-legal solution to this growing problem. While the company's system can't stop the CP4.2 from failing, it reroutes fuel flow leaving the pump so that—in the event of a failure—you're not buying fuel rails, lines, and an expensive set of piezo injectors on top of replacing the CP4.2.

To shine some light on how and why the CP4.2 fails, we'll walk you through the 6.7L Power Stroke's factory high-pressure fuel pump's operation. Then we'll illustrate how S&S Diesel Motorsport's Bypass Kit changes fuel flow in order to guard against extensive CP4.2-related damage. Trust us—if you want your '11-present Super Duty to keep moving mountains every day, this is the cheapest insurance plan on the market.

In 6.7L Power Stroke applications, the CP4.2 is located at the front of the lifter valley, is gear-driven by the engine's camshaft, and is timed to both the camshaft and crankshaft. Of a two-cylinder design, each piston is driven up and down in its respective bore courtesy of a mainshaft known as the CP4.2's camshaft. The lobes on the pump's camshaft are offset 180 degrees from each other, and each piston is actuated two times per crankshaft revolution.

Despite its high efficiency and compact packaging, when air or debris is introduced into the CP4.2 the pistons have a tendency to rotate, often 90 degrees, within their cylinder bores. Since the camshaft lobes' link to the piston is dependent on a roller tappet running parallel with the lobe, once the piston and corresponding roller tappet begin making contact with the cam at a perpendicular angle, catastrophic damage is imminent. Unfortunately, there is no provision (such as a keyway) to keep the piston from rotating within its cylinder.

When you add the rotated piston's improper operational relationship with the cam to the severe pressure fluctuations that occur in highly aerated, highly pressurized fuel, the roller tappet inevitably makes contact with the cam lobe. With repetitive contact being made between the two components, both quickly begin to break down.

While there is obviously a lot going on in this diagram, focus your attention on the yellow section that represents low-pressure fuel. As the roller tappet that's pressed into the bottom of the piston and its corresponding cam lobe begin to disintegrate, shards of metal are mixed in with the high-pressure fuel bound for the CP4.2's high-pressure outlets, which routes the contaminated diesel through the rails and into the injectors.

Specifically designed to keep the fuel injectors, rails, and lines unharmed in the event of a CP4.2 failure, the S&S Diesel Motorsport disaster prevention system offers utmost peace of mind to '11-present 6.7L Power Stroke owners. The all-inclusive CP4.2 Bypass Kit utilizes a billet-aluminum adapter block to effectively reroute the bottom end flow of fuel from the camshaft area of the pump back to the tank for a second round of filtration. And because the system does not alter the injection system in any way, emissions are unaffected and the CP4.2 Bypass Kit is 100 percent CARB compliant.

Thanks to the CNC-machined adapter block, should the CP4.2 begin to fail, all contaminated, shrapnel-laced fuel will be forced back to the tank, pulled through both fuel filters again, and sent through the lift pump once more before reentering the CP4.2. Prior to installing the adapter block in the CP4.2, the two smaller O-rings pictured (left) are installed on the supply tube portion. The larger pair of supplied O-rings are installed on the fuel control actuator (FCA, or MPROP) prior to installing the FCA in the adapter block.

Installing the CP4.2 Bypass Kit calls for the removal of both the upper and lower intake plenums to gain access to the CP4.2. In order to simplify reinstallation of the lower intake plenum once the bypass kit is installed, S&S recommends cutting down the leg on the bottom side of the cast-aluminum piece (shown). For better clearance around the FCA connector with the aforementioned adapter block in place, the company also recommends grinding down the casting flash rib located on the throttle valve side of the lower intake plenum.

Saving both time and labor, the CP4.2 doesn't have to be removed for the bypass kit install. Instead, the circlips for the supply and return fittings are removed (yellow arrows), and the FoMoCo-stamped T-bar is pulled and discarded (though its 10mm flanged head bolt is retained). From there, the factory hard supply line feeding the CP4.2 is pulled (red arrow). The FCA is also unplugged and removed from the CP4.2 at this point in the install.

In addition to the adapter block, fresh O-rings, and FCA fasteners, the CP4.2 Bypass Kit comes with a section of braided stainless fuel supply line. It integrates into the fuel system by tapping into the factory hard supply line (by way of cutting it square and thoroughly deburring the cut edges) near the secondary fuel filter base. The opposing compression fitting end threads into the adapter block on the CP4.2.

To allow the included braided stainless fuel line to rotate within the plastic factory hard line brackets near the secondary fuel filter base, the bracket nearest the braided stainless hose has to be slightly modified. Specifically, the "Christmas tree" post closest to the bracket mounting hole has to be cut off of the bottom side.

Once the bypass kit is in place, special care should be taken so that the braided stainless supply line has proper clearance near the lower intake plenum it should be routed under, the turbocharger, and the driver-side exhaust manifold heat shield. Then everything removed during disassembly can be reinstalled. By preserving the life of the injectors, rails, and lines in the event of a CP4.2 failure, S&S Diesel Motorsport's $360 bypass kit provides an insurance policy that can save you thousands of dollars if your CP4.2 ever decides to head south.

Unfortunately, thousands of CP4.2 pumps end up as paperweights. It doesn't happen to every '11-present diesel-powered Super Duty on the road, but the potential is always there. For truck owners who've experienced the calamity firsthand, the worst part is that no warning signs surface before the CP4.2 checks out. Instead, the engine suddenly stumbles and dies, never to restart.

What You Can Do to Help Prevent a CP4.2 FailureOf course, considerable relief comes with the addition of the S&S Diesel Motorsport CP4.2 Bypass Kit, but proper maintenance goes a long way, too. This is especially true on all modern-day common-rail diesel engines. Regular fuel filter changes (every other oil change is good practice) and fully priming the fuel system prior to starting the engine after those filter changes is the best way to guard against air infiltrating the high-pressure system. Note that all 6.7L Fords are equipped with a frame-mounted primary filter as well as a secondary filter located up on the engine. As for ensuring that only quality, clean fuel makes it into your tank, fill up at high-traffic filling stations that are regularly being resupplied with fresh diesel.